Background

In a standard configuration, the NameNode is a single point of failure (SPOF) in an HDFS cluster. Each cluster has a single NameNode, and if that host or process became unavailable, the
cluster as a whole is unavailable until the NameNode is either restarted or brought up on a new host. The Secondary NameNode does not provide failover capability.

The standard configuration reduces the total availability of an HDFS cluster in two major ways:

In the case of an unplanned event such as a host crash, the cluster is unavailable until an operator restarts the NameNode.

Planned maintenance events such as software or hardware upgrades on the NameNode machine result in periods of cluster downtime.

HDFS HA addresses the above problems by providing the option of running two NameNodes in the same cluster, in an active/passive configuration. These are referred to as the active
NameNode and the standby NameNode. Unlike the Secondary NameNode, the standby NameNode is hot standby, allowing a fast automatic failover to a new NameNode in the case that a host crashes, or a
graceful administrator-initiated failover for the purpose of planned maintenance. You cannot have more than two NameNodes.

Implementation

Quorum-based Storage

In order for the standby NameNode to keep its state synchronized with the active NameNode in this implementation, both nodes communicate with a group of separate daemons called
JournalNodes. When any namespace modification is performed by the active NameNode, it durably logs a record of the modification to a majority of the JournalNodes. The standby NameNode is capable of
reading the edits from the JournalNodes, and is constantly watching them for changes to the edit log. As the standby Node sees the edits, it applies them to its own namespace. In the event of a
failover, the standby ensures that it has read all of the edits from the JournalNodes before promoting itself to the active state. This ensures that the namespace state is fully synchronized before a
failover occurs.

In order to provide a fast failover, it is also necessary that the standby NameNode has up-to-date information regarding the location of blocks in the cluster. In order to achieve this,
DataNodes are configured with the location of both NameNodes, and they send block location information and heartbeats to both.

It is vital for the correct operation of an HA cluster that only one of the NameNodes be active at a time. Otherwise, the namespace state would quickly diverge between the two, risking
data loss or other incorrect results. In order to ensure this property and prevent the so-called "split-brain scenario," JournalNodes only ever allow a single NameNode to be a writer at a time.
During a failover, the NameNode which is to become active simply takes over the role of writing to the JournalNodes, which effectively prevents the other NameNode from continuing in the active state,
allowing the new active NameNode to safely proceed with failover.
Note: Because of this, fencing is not required, but it is still useful; see Enabling HDFS
HA.

Automatic Failover

Automatic failover relies on two additional components in an HDFS: a ZooKeeper quorum, and the ZKFailoverController process (abbreviated as ZKFC). In
Cloudera Manager, the ZKFC process maps to the HDFS Failover Controller role.

Apache ZooKeeper is a highly available service for maintaining small amounts of coordination data, notifying clients of changes in that data, and monitoring clients for failures. The
implementation of HDFS automatic failover relies on ZooKeeper for the following functions:

Failure detection - each of the NameNode machines in the cluster maintains a persistent session in ZooKeeper. If the machine crashes, the ZooKeeper
session will expire, notifying the other NameNode that a failover should be triggered.

Active NameNode election - ZooKeeper provides a simple mechanism to exclusively elect a node as active. If the current active NameNode crashes, another
node can take a special exclusive lock in ZooKeeper indicating that it should become the next active NameNode.

The ZKFailoverController (ZKFC) is a ZooKeeper client that also monitors and manages the state of the NameNode. Each of the hosts that run a NameNode also
run a ZKFC. The ZKFC is responsible for:

Health monitoring - the ZKFC contacts its local NameNode on a periodic basis with a health-check command. So long as the NameNode responds promptly with a
healthy status, the ZKFC considers the NameNode healthy. If the NameNode has crashed, frozen, or otherwise entered an unhealthy state, the health monitor marks it as unhealthy.

ZooKeeper session management - when the local NameNode is healthy, the ZKFC holds a session open in ZooKeeper. If the local NameNode is active, it also
holds a special lock znode. This lock uses ZooKeeper's support for "ephemeral" nodes; if the session expires, the lock node is automatically deleted.

ZooKeeper-based election - if the local NameNode is healthy, and the ZKFC sees that no other NameNode currently holds the lock znode, it will itself try to acquire the lock. If it succeeds, then it has "won the election", and is responsible for running a failover to make its local NameNode active. The
failover process is similar to the manual failover described above: first, the previous active is fenced if necessary, and then the local NameNode transitions to active state.

General Questions about HDFS HA

What does the message "Operation category READ/WRITE is not supported in state standby" mean?

In an HA-enabled cluster, DFS clients cannot know in advance which NameNode is active at a given time. So when a client contacts a NameNode and it happens to be the standby, the READ or
WRITE operation will be refused and this message is logged. The client will then automatically contact the other NameNode and try the operation again. As long as there is one active and one standby
NameNode in the cluster, this message can be safely ignored.

If an application is configured to contact only one NameNode always, this message indicates that the application is failing to perform any read/write operation. In such situations, the
application would need to be modified to use the HA configuration for the cluster. The Jira HDFS-3447 deals
with lowering the severity of this message (and similar ones) to DEBUG so as to reduce noise in the logs but is unresolved as of October 2018.

If this documentation includes code, including but not limited to, code examples, Cloudera makes this available to you under the terms of the Apache License, Version 2.0, including any required
notices. A copy of the Apache License Version 2.0 can be found here.